One of the challenges in wind turbine blade design is the provision of effective lightning protection systems. Such systems generally take the form of a lightning receptor provided on the exterior of a blade, usually at the tip end of the blade, the receptor coupled to a lightning down conductor located inside of the blade. The receptor acts to receive a lightning strike, which is conducted through the down conductor to the blade root end, at which point the lightning protection system is coupled to a ground connection provided on the greater wind turbine tower structure.
Various different types of connection systems are known for the connection of a lightning down conductor to a suitable ground connection at a blade root end, in particular to route a lightning strike around any sensitive blade components at the blade root, e.g. a pitch system. One known system is that of a spark gap solution, where a lightning down conductor is coupled to an external conductive plate spaced from the blade root end. A flexible arm extends from the wind turbine hub or nacelle, and is provided adjacent the external plate. A spark gap between the plate and a conductive element in the arm provides a conductive path to ground for a lightning strike in the wind turbine blade, the path to ground avoiding the root end components of the blade.
An alternative system is described in German patent application number DE 10 2008 045 939, which discloses a lightning bypass system wherein a lightning current can be routed through a securing T-bolt at the blade root end. The T-bolt passes through an aperture in the blade pitch system into the internal space of the wind turbine hub, wherein a lightning current is arranged to couple to a subsequent ground connection, e.g. through a subsequent spark gap connection. However, while the aperture through the pitch system may be provided with an insulating material between the T-Bolt and the pitch system, the use of such an arrangement will present problems due to ingress of moisture into spaces between the system interfaces, resulting in the formation of artificial spark gaps between the pitch system components and the lightning path to ground. Such spark gaps may result in damage to the relatively sensitive components of the wind turbine system in the event of a lightning strike.
Accordingly, it is an object of the invention to provide a wind turbine blade lightning bypass system which can more effectively conduct a lightning strike to ground without damaging sensitive blade components.